# c3_example.py
def get_age():
    #global age
    while True:    
        str_age = input('please input your age\n')
        if str_age.isdigit()== True:
            age = int(str_age)
            return age
        else:
            print('age must be digit')
age = get_age()
if age > 20:
    print('you are too old')
else:
    print('you are so young')
 
